A Biosynthetically Inspired Synthetic Route to Substituted Furans, and its Application to the Total Synthesis of the Furan Fatty Acid F5 A thesis submitted in partial fulfilment of the requirements of the degree Doctor of Philosophy Robert Jack Lee Supervisors: Dr Gareth J. Pritchard and Dr Marc C. Kimber 1 Thesis Abstract Dietary fish oil supplementation has long been shown to have significant health benefits, largely stemming from the anti-inflammatory activity of the ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) present in fish oils. The anti-inflammatory properties of these fatty acids has been linked to beneficial health effects, such as protecting the heart, in individuals consuming diets rich in fish, or supplemented with fish oils.1 These effects are highly notable in the Māori people native to coastal regions of New Zealand; the significantly lower rates of heart problems compared to the inland populous has been attributed to the consumption of the green lipped mussel Perna Canaliculus. Commercially available health supplements based on the New Zealand green lipped mussel include a freeze-dried powder and a lipid extract (Lyprinol®), the latter of which has shown anti- inflammatory properties comparable to classical non-steroidal anti-inflammatory drugs (NSAIDs) such as Naproxen.2 GCMS analysis of Lyprinol by Murphy et al. showed the presence of a class of ω-4 and ω-6 PUFAs bearing a highly electron rich tri- or tetra-alkyl furan ring, which were designated furan fatty acids (F-acids).3 Due to their instability, isolation of F- acids from natural sources cannot be carried out and a general synthetic route toward this class of natural products was required. To accomplish this, the biosynthesis of F-acids was mimicked by utilising an oxidation of 1,3- dienes, followed by a dehydration/aromatisation to generate the heterocyclic furan ring. Singlet oxygen was chosen as the means of oxidising the conjugated dienes giving endoperoxides. To mimic the biological aromatisation of the peroxide intermediates the Appel reagent was chosen and, in a novel application of the reagent, was exploited as a mild, metal free method of dehydrating the cyclic peroxides to their corresponding furans. 2 The biomimetic furan synthesis was applied toward a selection of 1,3-diene substrates bearing a range of pre-installed functionalities and substitution patterns including alkyl, aryl, alkenes, cyclopropyl rings, silyl ethers, and esters, alongside being applied to the total synthesis of the furan fatty acid F5. A brief exploration of the possibility of performing the aromatisation reaction under catalytic conditions was carried out, to determine whether endoperoxides could be converted to furans without needing a stoichiometric quantity of Appel reagent, by harnessing a catalytic quantity of triphenylphosphine oxide and regenerating the active P(V) species via reaction with oxalyl chloride. Furthermore, an optimisation study was carried out using a simple design of experiments procedure to ascertain the ideal conditions for carrying out the Appel-type dehydration of endoperoxides. Finally, the scope of the reaction sequence was expanded to be performed in a continuous flow reactor, with telescoping of the singlet oxygen diene oxidation and Appel-type aromatisation to increase oxidation yields and to omit the requirement for isolation of peroxide intermediates, and was applied to the synthesis of a selection of 2,5-diaryl furan motifs. 3 Acknowledgements Firstly, I would like to thank my two fantastic supervisors, Dr Marc Kimber and Dr Gareth Pritchard for giving me the opportunity to work on this project, and for your continued advice, support, and encouragement you’ve provided over the past years. From undergrad labs and projects, you’ve constantly been teaching me new skills, tricks of the trade, and making me think and get excited about chemistry, and that’s only continued throughout my PhD. Your patience has been unfaltering, and you’ve both been great supervisors, but also great friends and drinking partners for the past few years (although I’m not sure I’ll ever be able to hold my drink like Gaz…or pretend to like Marc!). To all the other researchers over the years in F001 and F009, you’ve been a constant source of support, amusement, and occasionally frustration, but my time in the lab would not have been the same without you all. Thanks to Nat for supporting me since my 3rd year undergrad project, being someone to bounce ideas off, and being my first port of call when I’ve needed to rant and moan. Dani, your childlike innocence was constantly there, trying to stop me from being a miserable old bugger, even when (as I told you early on that it would) you started to become one yourself…you managed to hold out far longer than I’d ever have expected though. Conversely, Harding and Monaghan, you were the likeminded miserable old buggers I needed around to fuel and share in my crotchety state of mind, especially in those last few months. Finally, thanks to Vlod for being one of the most ridiculous people I’ve ever met and for never failing to make me laugh, whether intentionally or purely through one of your many questionable choices of clothing or haircut. I’d like to acknowledge all the technical staff and academics who’ve helped to support my research; from teaching me to use and maintain instruments, to making suggestions on reactions I could try in my work. 4 Eleanor, you have managed to pull me away from my work, albeit briefly, and given me something other than the lab to look forward to each day. I wouldn’t have got through everything without your support, your grounding, and your pushing me to keep going. Mum and Dad, you’ve been there through everything and your support has been invaluable; I’m not going to be a student for much longer! I hope I can make you proud. 5 Table of Contents Thesis Abstract 2 Acknowledgements 4 Abbreviations 7 1. Introduction 9 1.1 Heterocyclic Compounds 9 1.2 Furans 12 1.3 Inflammation 19 1.4 The Effect of Fatty Acids on Inflammation 24 1.5 Lyprinol® and Furan Fatty Acids 28 1.6 Previous Synthetic Routes to Furan Fatty Acids 33 2. Aims 41 3. Results and Discussion 43 3.1 The Dehydration of Endoperoxides as a Route to Furans 43 3.2 Reaction Optimisation by Design of Experiments 66 3.3 Catalytic dehydration of 1,2-dioxines using a modified Appel approach 70 3.4 Total Synthesis of Furan Fatty Acid F5 76 3.5 The application of flow chemistry for the preparation of endoperoxides and furans 86 4. General Conclusions 110 5. Future Work 112 6. Experimental 114 6.1 General Information 114 6.2 Synthetic Procedures 116 6.2.1 Diene Precursors 116 6.2.2 Dienes 127 6.2.3 Endoperoxides 148 6.2.4 Furans 162 6.2.5 Furan Fatty Acid F5 Total Synthesis 186 5.0 References 192 6.0 Publications 201 7.0 Appendix 1 – Bis-Peroxide NOE 202 6 Abbreviations AA Arachidonic acid BOC tert-Butyloxycarbonyl br Broad BuLi Butyllithium COPD Chronic obstructive pulmonary disease COX Cyclooxygenase d Doublet DHA Docosahexaenoic acid DIBAL-H Diisobutylaluminium hydride DIEA N,N-Diisopropylethylamine (Hünig’s base) DMAP Dimethylaminopyridine DMF N,N-Dimethylformamide DMP Dess-Martin periodinane DPA Docosapentaenoic Acid EPA Eicosapentaenoic Acid Eq. Equivalents ESI Electrospray ionisation F-Acid Furan fatty acid FAME Fatty acid methyl ester GC-MS Gas chromatography-mass spectrometry GI Gastrointestinal h Hours Hz Hertz IR Infrared J Coupling constant KHMDS Potassium hexamethyldisilazide (Potassium bis(trimethylsilyl)amide) 7 MS Mass spectrometry m/z Mass to charge ratio NaHMDS Sodium hexamethyldisilazide (Sodium bis(trimethylsilyl)amide) NMR Nuclear magnetic resonance NSAID Non-steroidal anti-inflammatory drug PC Principal Component PCC Pyridinium chlorochromate PDC Pyridinium dichoromate PLA2 Phospholipase A2 PUFA Polyunsaturated fatty acid q Quartet s Singlet SPE Solid Phase Extraction t Triplet TBS tert-butyldimethylsilyl THF Tetrahydrofuran TLC Thin layer chromatography r.t Room temperature UV Ultraviolet 8 1. Introduction 1.1 Heterocyclic Compounds Heterocycles are a family of compounds which contain a cyclic system bearing one or more non-carbon or hydrogen elements (such as oxygen, nitrogen, or sulfur) in the ring.4 Heterocyclic compounds have wide variations in ring size, type of heteroatom(s) present, the number of heteroatoms in the ring, and the degree of unsaturation. Fused heterocyclic ring systems are also known, where one or more of the rings contain a heteroatom. Heterocyclic rings are a common feature in a plethora of organic molecules, ranging from natural products to synthetic structures. They are present in almost every aspect of modern life, including pharmaceuticals, agrochemicals, polymers, cosmetics, dyes, electronics, and food additives.5, 6 As a result, compounds containing heterocyclic units hold the focus of huge research efforts into novel methods for their synthesis; over the past century, heterocyclic chemistry has grown to be one of the largest subdivisions of synthetic organic chemistry.5 Many heterocyclic compounds possess interesting biological properties, a feature which owes to the unique geometric properties of the cyclic systems and the electron distributions which are influenced by the presence of heteroatoms.5 These features allow for the effective binding of heterocyclic molecules to biological targets such as ligand receptors and enzymes, making them highly attractive candidates for drug molecules. This is perhaps best expressed in the sales of pharmaceuticals, in which eight of the ten highest selling small molecule drugs in 2013 contained one or more heterocyclic components, netting over $33 billion in sales.7 9 Figure 1. The three highest selling small molecule drugs in 2013, with heterocyclic components highlighted in red.7 Heterocyclic compounds may be divided into two general classes; those that exhibit aromaticity, and those that do not.